The present invention relates to a programmable controller which performs sequence control of a system, more particularly it relates to a control technique for displaying plural abnomalities occurring in a system to be controlled.
As is well known, a programmable controller is composed so that the sequence control of a system can be realized by repeatedly executing in sequence a group of instructions stored in a user's program memory.
As a system for which sequence control is to be applied, a conveyor system, for example, is now considered. In this system, articles are placed on a conveyer belt and they are transported from one side to the other side and vice versa. The control operation of a programmable controller to be applied to such a system can be summarized as follows. Namely, the controller upon receipt of detection signals from detectors, which detect articles on the conveyer belt, generates instruction signals whereby plural motors which drive the conveyor belt are caused to perform such operations as start, stop, normal rotation and reverse rotation, etc. in a fixed sequence. The instruction signals are generated as a result of logical operation of input condition and output conditions (these conditions are referred to as input/output data). Also, within the conveyer motors, there is a group of motors wherein the movements are performed simultaneously and another group of motors wherein the movements are not performed simultaneously. In any case, when an instruction signal which directs a movement contrary to the classfication of the group within which the motor is located is generated, an abnormality of the system is brought about. This means that a logical abnormality has occurred in a stage of logical operation at the side of the programmable controller.
Conventional programmable controllers have been designed to optionally set an abnormality detection program in the user's program memory as part of a user program so that an abnormality of a system to be controlled can be detected by the user program and the abnormality thus detected can be acknowledged. In other words, it is conventionally arranged as follows. When detecting the logical abnormality by the execution of an instruction of the abnormality detection program, the machine number of the input/output device (for example, is issued as abnormality information.
However, in the case of a conventional programmable controller, if plural abnormalities are detected in a stage of the execution of a user program, said abnormality information to be issued is such that it corresponds to only one of the plural abnormalities. Moreover, the controller has been designed so that such abnormality information cannot be reset unless the cause of the abnormality is eliminated.
In other words, in a conventional programmable controller, when plural abnormalities occur at the same time, the user of the controller is forced to use a troublesome abnormality recovering process, as described below. First of all, the user stops the controller and performs a recovery process to remove the cause of the abnormality, thereby putting the device back into operative condition. Then, the next abnormality information is presented and in this stage the user notices that another abnormality has occurred. Thereupon, the user begins to remove the cause of the next abnormality. In this way, the user must repeat operations in order to eliminate all the causes of plural abnormalities.
As is well known, for those abnormalities which occur at the same time in a system, some are caused by completely independent causes, while others are related to each other and thus one abnormality induces other abnormalities in many cases. For example, in the above mentioned conveyer system, if an abnormality occurs in a detector, it could cause the issue of an instruction signal which causes a conveyer motor to abnormally operate. In this case, the trouble can be easily solved by simply recovering the faulty detector. In such a case, a program exists such that only one abnormality information is issued, and moreover it is unknown which one of a detector abnormality and that of a conveyor motor abnormality is presented first, due to the properties of the programmable controller. When the abnormality of the conveyer motor is presented earlier, the user is forced to carry out the aforementioned troublesome recovery processing.
As a countermeasure for such troublesome processing, a method whereby all abnormalities which occurred are displayed for the judgement of a user may be considered. However, this idea would not be practical since it would require a large and expensive device.